Trucks, vans, cargo trailers, and even train box cars tend to present a trailing edge which is box-shaped, with a flat vertical tail and four sharp corners meeting a top side, bottom side, left side (driver's side) and right side (passenger's side) of the cargo carrying portion of the vehicle. It is well known that this shape creates substantial aerodynamic drag as the vehicle moves through the air because the air tends to be compressed as it encounters the leading edge of the vehicle (cab, truck, hood/windshield, train engine, etc.), flows around the top, bottom and sides of the vehicle, and then is suddenly allowed to decompress as it passes by the rear corners of the vehicle. This sudden decompression creates an eddy current effect known as cavitation, in which a portion of the air actually travels forward into a zone of lower pressure situated on the rear vertical surface of the vehicle. This causes a pulling backwards on the vertical rear surface, which increases the drag coefficient of the vehicle.
Yet, the flat rear vertical surface of these types of cargo transportation vehicles remain popular because they facilitate docking to a door in a warehouse for easy loading and unloading of the vehicle. Some of these surfaces are configured with two door panels hinged at the outer side edges to swing open, and others are provided with a multi-panel roll-up door that lifts and stores near the interior ceiling of the cargo storage area.